You should never attempt to directly reference an
itemRenderer in any list component. That's because we only generate
itemRenderers for the visible rows of the list plus a couple extra
for buffering purposes. The itemRenderers are the recycled as you
scroll through the list. The Flex docs talk about this extensively.

If you want to change an itemRenderer's alpha (or any other
characteristic) you do it from within the itemRenderer code, doing
so based on some value of the data. Let's say your itemRenderer is
an MXML file based on Canvas and in your data record you want the
alpha value set from a field called "price". You might do it like
this:

<mx:Canvas xmlns:mx="..." alpha="{data.price == 0 ? .25 :
1}" >

So if the price is zero, then the alpha will be 0.25
otherwise it will be 1.

Sorry, but sometime we want to aceess/control the item inside
the itemRenderer.
For example, imagine I have a video player (main.mxml) where
this object class have an itemRenderer list of scenes
(videoScene.mxml).
What if I want the video while it is been play, it reach a
time (let say 15 secs) where I want the selected scene of the video
(that have the value of 15 secs) to be highlighted to the user
that, the video is entering/playing this* scene?
I cant do something similar as what you guys suggested here,
right?

An itemRenderer should be handling 1 thing from a list of
similar things. So let's suppose you have a collection of 20 items
where each item in the list has information to display a video. If
the list control is sufficently large it is possible to show all 20
videos at once. But suppose it can show only 5 videos - 5 items -
at once. There would only be 5 or 6 itemRenderers then. As you
scroll through the list, those 5 or itemRenderers will be given new
data - different items to display. So the itemRenderer must use the
data it is given to show the video content.

While a video is playing in an itemRenderer the code
supporting the itemRenderer can take its direction from anything
the video data, or the data for the record itself, has to offer.

If you itemRenderer has a Timer that goes off after 15
seconds, then the itemRenderer code can respond to that and change
its color, for example. But keep in mind, the user could scroll
that itemRenderer out of view where the list control will then
repopulate that very same itemRenderer with new data. This is how
itemRenderers were designed to work in Flex.

To sum up: the itemRenderer can do whatever it wants as long
as the data for the record it is given has the information.